卷管铺设复合管环焊缝的工程临界评估方法研究

Subsea pipelines is an important part of offshore oil and gas development and production systems，and development of deepwater reeling installation technology for clad pipes is needed in order to develop and use the oil and gas resource in the South China Sea. During reeling installation and high-temperature and high-pressure service, the pipelines undergo several high plastic strain cycles which may cause weld flaws in girth welds to grow up, and threaten the safety of pipelines. However, current design standards and structural integrity codes do not provide specific guidance how to perform engineering critical assessment (ECA) with bi-metallic girth weld flaws in clad pipelines. The use of current standards and codes after modification often results in over-conservative assessment results, which will cause excessive flaws repair. In the study, based on elastic-plastic fracture mechanics, fatigue mechanics, damage mechanics and computational science, the coupling effect of complex girth weld strength match, internal pressur, weld misalignment and welding residual stress on crack drive force is investigated considering cyclic plastic strain and estimation methods of reference stress for surface flaw and embedded flaws with different deepth, the construction method for crack growth resistance curves of embedded flaws and verification test methods for ECA procedure will be developed for girth welds in clad pipelines under cyclic plastic strain condition. At last, the ECA methods for girth weld flaws in clad pipelines for reeling installation will be estalished.

海底管线是海上油气田开发生产系统的重要组成部分，发展深水复合管卷管铺设技术是我国开发南海油气资源的迫切需求。在卷管铺设和高温高压运行阶段，管线经历多次塑性应变循环，可能引起环焊缝焊接缺陷长大，危及管线安全。现有完整性规范没有专门针对复合管环焊缝的工程临界评估（ECA）方法，而对现有方法改良后使用的评估结果往往过于保守，导致过度的缺陷返修，带来巨大的经济浪费。本项目拟针对卷管铺设复合管环焊缝焊接缺陷，综合利用弹塑性断裂力学、疲劳力学、损伤力学与计算科学等理论与方法，结合数值模拟、试验测试与理论分析等手段，考虑循环大塑性应变条件，研究环焊缝强度匹配、内压、焊接错边和焊接残余应力的耦合影响机制，分别研究建立表面缺陷、近表面埋藏缺陷和深埋缺陷的参考应力计算方法、埋藏缺陷裂纹阻力曲线构造方法和ECA程序有效性验证方法，并与现有规范的ECA评估体系相耦合，形成完整的卷管铺设复合管环焊缝ECA评估方法。

大变形海底管道作为长距离深海油气运输的主要载体，其性能非常重要。Reel-lay卷管铺设方法是在传统的S型和J型铺管技术发展而来的可以陆上焊接的铺管技术，在卷管和铺设过程中管道会受到超过屈服强度的较大塑性变形，超出了常规意义上金属材料的力学应用范围，因此需要对管道的安全性进行工程临界评估(ECA)。含缺陷金属材料在塑性阶段的力学评估研究对于大变形海底管道的应用至关重要。工程临界评估常用方法包括EPRI法、参考应力法和参考应变法。目前国际上现有的ECA评估标准大多是以参考应力法作为理论基础，按照该评估方法得到的评估结果往往过于保守，对于实际工况的考虑不够完善。本研究在现有国际标准的基础上考虑了服役过程中内压对管道断裂评估的影响，也考虑了复合管内衬层、匹配焊缝、坡口角度、根部间隙等因素对断裂评估的影响，对参考应力法和参考应变法予以改进，使评估的条件更加接近实际工况。